Frequency modulation signal generator



Aug. 8, 1944. A. w. STEWART FREQUENCY MODULATION SIGNAL GENERATOR 2 Sheets-Sheet 1 Filed Feb. 20, 1943 m rama. u mum.: mozmhoxmm ...5L :MDL oxnmoLwEm ar...:umo u.: mimi.; zorrowzzau zorfuuzzou zorrawzzou mmrmw: NA 55u: We EEWIN: W

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Patented Aug. 8, 1944 FREQUENCY MoDULA'rIoN SIGNAL GENERATOR Arthur W. Stewart,"'l)etroit, Mich., assignor to Detrola Corporation, Detroit, Mich., a corporation of Michigan Application February Z0, 1943, Serial No. 476,545

(Cl. Z50-36) 8 Claims.

My invention relates to a frequency modulated signal generator incorporating a continuously variable frequency modulated oscillator in which the frequency deviation is independent of the carrier frequency.

In designing radio sets for operation on frequency modulation transmitted signals, and in testing and experimenting with such sets and with frequency modulation equipment generally, it is necessary to have in a laboratory for such purposes a frequency modulation signal generator. Such'generators have been built in the past and operated for such purposes. All such signal generators of which I am aware, that use reactance tubes for frequency modulators, however, have (a) used the heterodyne method of obtaining the output frequency when tuning over a wide range, or, (b) have used a very narrow band variable oscillator, with frequency multipliers to obtain the desire output band of frequencies resulting in a narrow output tuning range and therefore having many tuning ranges which are selected by a band switch in order to cover several bands of frequencies which should be covered in a single band. In the case of (a) the frequency deviation is constant over the tuning range because the frequency modulated oscillator is of a fixed frequency and the deviation is an exact multiple of the fixed oscillator deviation over the tuning range. Be,- cause of the very nature of the method used to obtain the output frequency of the signal generator the output contains signals of various harmonics and spurious frequencies which are confusing to the user.

It is well known that when using reactance tubes for frequency modulators, the amount of deviation obtained at a given frequency can be changed by changing the L. C. ratio of the oscillator tank circuit. Therefore, in designing a frequency modulated signal generator which will cover a wide range of frequencies in one continuous range, the deviation will vary'over the band as the tuning condenser is varied, and provision must be made to reset the deviation to the desired amount. Ordinarily this would require a calibrated deviation meter incorporated in the signal generator which measures the output deviation of the oscillators or amplifiers and a manual control to reset the deviation to the desired value.

The object of this invention is to produce a frequency modulated signal generator covering a wide band of frequencies with the use of frequency multipliers in one continuous band and of producing a selected amount of frequency deviation which is independent of the carrier frequency throughout the band, without the use of frequency deviation measuring devices incorporated within the signal generator and with the output signal voltage practically free of any spurious frequencies and harmonics.

A feature of this invention is the provision ofA an audio oscillator and amplifier having a variable gain control which is ganged to the tuning condensers of the oscillator and frequency multipliers of a frequency modulation signal generator, thereby controlling the gain of the audio amplier in accordance with the angular rotation of the tuning condensers, maintaining the required amount of audio voltage to the reactance tubes to maintain a constant R. F. oscillator frequency deviation.

In the drawings, Figures 1 and 2 together constitute a circuit diagram of a signal generator in accordance with my invention.

As the circuit is drawn in conventional form and the values of the various elements and types of various tubes are indicated, little explanation of the circuit is necessary to enable those skilled in the art to construct it beyond an explanation of the particular features with which my invention is particularly concerned.l The circuit shown is for a signal generator operable over a continuous range of from 19 to 41 megacycles frequency modulated by means of reactance tubes, an audio signal impressed thereon, which may be manually varied to give a frequency deviation of from 10 to 100 kilocycles and may be tuned to give a signal of any desired frequency throughout the band while automatically maintaining a constant deviation of the selected value at any frequency.

I employ an audio oscillator and amplifier tube I which may be of the 12SN7 type, and is provided wth a switch 2 for producing a modulation frequency of 400 cycles or of 1000 cycles, or can use any other modulation frequency as may be desired. A variable cathode biasing resistor 3 permits a variation of the amplitude of the output signal from the tube l. A resistor 4 in the output circuit of the tube l has a contact 5 which may be manually set to give a frequency deviation in the output of the generator anything from 10 to 1000kilocyc1es. The resistance 3 is adjusted manually to provide a, deviation between these limits. An amplified audio signal from the tube I is fed through the circuits shown to the two push-pull and reactance tubes 6 and l which may be of the 12SA7 type.

lated oscillations are applied through a buffer tube I0, which may be of the 68H7 type, to a frequency tripler tube Il which may be of the 68H7 type, and in turn to a frequency doubler tube l2, which again may be of the 68H7 type. The output of the tube I2 is applied to tube Il which may be of the 8AG7 type, and which operates upon a saturated portion of its characteristic to produce a frequency modulated signal in its output of constant amplitude within over the frequency band resulting in the elimination of a vacuum tube voltmeter for measuring the output of the signal generator. The

- attenuator is used as the output load of the tube I3 and since the load is approximately 100 ohms. which is small compared to the reactance of the output capacity of tube il, the resulting fre quency characteristic is negligible.

The tuned circuits Il, Iland Il each has a variable condenser for tuning which are ganged together and also ganged with a variable contact I1 on Ia resistance i8 in the input circuit to the amplier portion of tube I. When the resistance 3 is set to a value to give a frequency deviation in the output of the signal generator which varies between and 100 kilocycles in accordance with the setting 'of contact I on resistance l, and when the contact l is set for the desired frequency deviation, this frequency deviation will be maintained constant throughout the range of frequencies to which the signal generator operates, which is 19 to 4l megacycles` for the particular circuit and values shown. At the high frequency end of the range of the signal-generator the contact II will be at the ground end of resistance Il and the resistance between contact -Il and ground will be increased as the frequency of the signal generated is decreased. The resistance I8 may be calibrated and the contact i1 arranged to include in the circuit the additional increments of resistance Il which may be necessary to accomplish this in any desired manner. For example, resistance il may be wound with varying amounts of resistance wound in linear sections for a given angular rotation to give the necessary amount 'of resistance, or the' contact l1 may becam operated to move at varying rates and thereby include varying amounts of resistance II in the circuit between contact l1 and ground, in accordance with-the amount necessary to accomplish the desired result. The curve or taper of the resistance control Il will follow exactly the capacity curve of the condenser used to tune the frequency modulated oscillator. 'The settings are not critical for results suillciently constant to provide perfectly satisfactory and valuable experimental data.

What is claimed is:

l. A frequency modulation signal generator comprising means for producing a signal of audio frequency, means for producing a signal of radio frequency, means for modulating the frequency of said radio frequency signal with said audio frequency signal, means for varying the frequency of said radio frequency signal, and means for maintaining the frequency deviation of said frequency modulated signal constant throughout the range of said radio frequency oscillator.

2. A frequency modulation signal generator comprising means for producing a signal of audio frequency, means for producing a signal of radio frequency, means for modulating the frequency of said radio frequency signal with said audio frequency signal, means for varying the frequency of said radio frequency signal, and means lfor maintaining the frequency deviation of said frequency modulated signal constant throughout the rangev of variation of said radio frequency oscillator, said means comprising means for varying the amplitude of the audio oscillations in proportion to the frequency of the radio frequency oscillations.'

3.A frequency modulation signal generator comprising an audio oscillator, a radio oscillator, means for modulating the frequency of said radio oscillator with said audio oscillations, means for varying the frequency of said radio oscillations. and means for varying the amplitude of said audio oscillations, said frequency varying and amplitude varying means being unicontrolled.

4. A frequency modulation signal generator comprising an audio' oscillator, first control means for varying the amplitude of said audio oscillations, a radio oscillator, means for modulating the frequency of said radio oscillations with said audio oscillations, and second control means for varying the frequency of said radio oscillations, said first and secondeontrol means being ganged for unicontrol.

5. A frequency modulation signal generator comprising an audio oscillator, manual means for varying the amplitude of said oscillations. first control variable means for controlling the ampli-- tude of said oscillations, a manual deviation control, a radio frequency oscillator, means for modulating the frequency of said radio frequency oscillations with said audio frequency oscillations, second control means for varying the frequency of said radio Afrequency oscillations, said first and second control means being ganged for unicontrol and means for maintaining the amplitude of the output signal from' said signal generator throughoutthe tuning range of said signal genconstant, whereby said signal generator emits -a frequency modulated signal wave of constant amplitude and constant deviation throughout its tuning range.

6. A frequency modulation signal generator comprising an audio oscillator, first control means for varying the amplitude of oscillations of 4said audio oscillator, a radio oscillator, second control means for varying the frequency of said radio oscillations, means for modulating the frequency of said radio oscillations with said audio oscillations, a frequency multiplier, and third control means for varying the frequency of said frequency multiplier, said ilrst, second, and third control means being ganged for unicontrol.

7. A frequency modulation signal generator comprising an audio oscillator, vnr'st control means for varying the amplitude of said audio oscillations, a radio oscillator, second control means for varying the frequency of said radio oscillations, said first and second control means being ganged for unicontrol, means for modulating the frequency of said radio oscillations with said audio oscillations, manual means for setting the frequency deviation of said signal generator to the desired value, said unicontrol means maintaining the frequency deviation at the same value erator.

8. A frequency modulation signal generator comprising means for producing a signalof audio frequency, means for producing a signal of radio frequency, means for modulating the frequency of said radio frequency signal with said audio frequency signal, means for varying the frequency of said radio frequency signal, and means for maintaining the frequency deviation of said frequency modulated signal constant throughout the range of variation of said radio frequency oscillator. said means comprising means for varying the amplitude of the audio oscillations at a varying rate so as to follow the variations in the fre- 5 quency of the radio frequency oscillations.

ARTHUR W. STEWART.

`Disclaimer.'

2,355,338.Arthur W. Stewart, Detroit, Mich. FREQUENCY MonUm'rIoN SIGNAL GENERATOR. Patent dated Aug. 8, 1944. Disclaimer iled May 5, 1950, by

theassignee, Newport Steel Corporation.

Hereb enters this disclaimer to claims 1, 2, 3, and 4 of said patent.

[ Gazette June 27', 1950.] 

